Replace is the default selection setting. So if it's the default setting, when would you ever use Replace outside of your initial selection? Say you have made a selection, added to it a few times and realized that that just wasn’t working. At this point you might want to start again, so Replace would come in handy.

Replacing a selection in Sketchbook Pro for desktop

Once a selection is made, tap , then tap-drag to capture a new selection.

Replacing a selection in Sketchbook for mobile devices

Once a selection is made, tap , then tap-drag to capture a new selection.